Lightweight composite materials hold great promise for the aircraft industry. Fiber composites provide a significant improvement in specific strength and stiffness over conventional metal alloys. Better specific strength and stiffness translates into weight savings, which translates into fuel savings and lower operating costs. Additionally, composites do not corrode like aluminum, and they are more resistant to fatigue.
Composite elements such as skins, stiffeners, frames and spars are joined together to form major components such as wings, fuselage and empennage. The composite elements may be bonded together with polymeric adhesive. In theory, adhesive bonds alone have sufficient strength and integrity to support loading of these components. Therefore, adhesive bonds should be able to greatly reduce the number of metal fasteners in the major components.
In practice, however, certain federal aviation regulations require substantiation that a bonded joint between any two primary structural components will carry a specified load with a maximum disbond (that is, where an entire bond line is missing). One solution to this lack of confidence in adhesively bonded joints has been to add metal fasteners. If an adhesively bonded joint fails, a metal fastener would continue holding the joint together.
The use of metal fasteners adds weight to aircraft components. The use of metal fasteners with composite structures also increases the time, cost and complexity of fabrication. High precision machines and complex procedures are used to drill through composite structures. Moreover, penetrations for fasteners provide unwanted paths for lightning strike and corrosion.
Weight is also added by plies of composite that are added around the drilled holes to satisfy requirements for by-pass bearing loads. The additional plies also increase cost of an aircraft. The presence of fastener holes also forces the selection of composite ply layup orientations that reduce the strength of panels and bonded joints (as compared to optimally designed panels and joints without fasteners).
It is believed that adhesive bonds alone, if properly designed, prepared and controlled, have sufficient strength and integrity to bond primary structures together. However, data proving consistency and reliability is unavailable, and current inspection techniques are inadequate to establish confidence in adhesive bonds.